This invention relates generally to power generation involving the combustion of gas fossil fuels, and more particularly to methods and apparatus for reducing pollutant emissions in heavy-duty gas turbine power generators.
As used herein, a 50 Hz F-Class heavy-duty gas turbine refers to a gas turbine having a rated ISO day firing temperature on the order of 2400° F. (1315° C.), the highest average working fluid temperature in the gas turbine from which work is extracted, measured at the inlet plane of first rotating, work-extracting turbine blade, or bucket. Levels of NOx emissions below 9 ppm, corrected to 15% oxygen, can be achieved in such gas turbines using known premixing technology along with further enhancements to Dry Low NOx (DLN) systems currently available in the 50 and 60 Hz F-Class turbine markets. Low emission gas turbines of this type provide bluff body flame stabilization via a combination and optimization of component geometries, a center fuel nozzle in the combustor that acts as a combustion flame stability anchor for the overall flame structure of the combustion system, a fuel and air staging system design that includes multiple fuel manifolds and introduction points in the combustor, and sealing between key mating components. At least one known gas turbine, the General Electric model MS7001FA gas turbine (available from General Electric Co., Fairfield, Conn.), already achieves less than 9 ppm NOx emissions operating in the 60 Hz power generation market. However, another known gas turbine, the GE MS9001 FA 50 Hz gas turbine, is currently guaranteed at less than 25 ppm NOx emissions.
Governmental legislation now being pursued or already in effect in several international locations such as Northern Italy and Spain will significantly limit the amount of NOx emissions allowed from 50 Hz heavy-duty gas turbines used in electrical power generation.
One way to reduce such emissions is to install a selective catalytic reduction (SCR) device on the exhaust system of the gas turbine plant. SCR devices can be costly to install and operate, require expensive process chemicals such as anhydrous ammonia on a continuous basis to function, and carry the environmental risk of ammonia emission as a by-product of their operation. Many countries prohibit the use of ammonia-based SCR devices, and in such countries, gas turbine operators must operate lower firing temperatures below intended design ratings to achieve emissions compliance. While lower NOx emissions can be achieved at firing temperatures below the originally intended design ratings of a gas turbine (a process known in the art as “derating” the turbine), the power output and efficiency of the plant are reduced, ultimately resulting in lost revenue opportunity for the power producer.
In many early versions of gas turbine NOx control technologies, in which minimum NOx levels on the order of 40 to 50 ppm are achievable, water injection is employed, in a manner known by those skilled in the art, to reduce NOx emissions. Further improvements in NOx emissions have been realized using various forms of DLN technology, however, systems employing this technology have inherent operational restrictions due to combustion instability and combustion dynamic pressures or acoustic noise. Also such systems can operate in low emissions mode only over a very limited gas turbine load range.